The objective of this research is to investigate the molecular mechanisms bacteria use to specifically interact with eukaryotic hosts. By taking advantage of the natural symbiosis between the Hawaiian squid Euprymna scolopes and its bacterial symbiont Vibrio fischeri, the proposed experiments will explore the role of bacterial motility during host colonization by a beneficial microbe. This natural host-bacteria system is amenable to modern genetic tools and in vivo analysis with confocal fluorescence microscopy. The specific aims of this research application are: 1) to characterize each type of flagellin during initial colonization of the squid light organ;and, 2) to identify the bacterial determinants, including those involved in motility, that govern the long-term persistence of V. fischeri in E. scolopes. In addition to elucidating the role of bacterial motility in colonization and persistence, the results of the proposed experiments will shed light on the link between motility and the expression of virulence genes found in many pathogenic bacteria. Furthermore, insight into the interactions between eukaryotic hosts and their natural symbiotic microbes will help us understand the impact that microbes have on our own health.